Sediment that erodes off hillslopes and enters rivers plays a key role in the short- and long-term evolution of river basins. In dryland environments dominated by surface runoff on hillslopes, it is challenging to determine or observe the impact of hillslope sediment supply to rivers because of the episodic and variable nature of erosion driven by rainstorms.

Michaelides and Singer investigate the dynamics of runoff-driven hillslope erosion in a dryland basin dominated by brief, high-intensity rainstorms where sediment moves infrequently. The researchers found that the amount and size of sediment delivered from hillslopes to the valley floor depend on interactions between hillslope angle and length and the intensity and duration of the rainstorm.

The researchers combined field measurements with numerical modeling to investigate how rainstorms produce runoff and erosion on hillslopes within a dryland basin situated in southeast Spain. They discovered that long slopes, in particular, were more sensitive to storm runoff and produced highly variable mass and sediment sizes in different rainstorms. Moreover, runoff during typically short storms produces sediment redistribution within hillslopes before being delivered to the river channel, resulting in grain size-selective sediment contributions to the valley floor. These findings have implications for the interpretation of basin-wide erosion rates derived from cosmogenic radionuclides.

After taking measurements of topography and sediment grain sizes on hillslopes and in the river channel at 29 valley cross sections and modeling various rainstorms over the basin, the researchers discovered a relationship between the grain sizes on the hillslope and in the river channel. Specifically, the results suggest that the larger sediments measured in the river bed, which are assumed to constitute the hydraulic roughness in the channel, are likely derived from the median sediment sizes eroded off the hillslope.

This discovery suggests that hillslope erosion in dryland environments is the primary contributor to characteristic channel roughness.

The results of this study have implications for understanding the long-term evolution of dryland basins. (Journal of Geophysical Research: Earth Surface, doi:10.1002/2013JF002959, 2014)

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